Foldable and washable textile-based OLEDs with a multi-functional near-room-temperature encapsulation layer for smart e-textiles

The study addresses the need for reliable, foldable, and washable textile-based OLEDs that can serve as display interfaces for wearable electronics in IoT applications. Conventional encapsulation processes often require high temperatures (~100 °C) that damage thermally sensitive OLEDs and ultrathin flexible substrates. Existing barriers typically fail to achieve the ultralow water vapor transmission rate (WVTR ~10⁻⁶ g m⁻² day⁻¹) required by OLEDs, and frequently lack sufficient flexibility (bending radius <5 mm or strain >1%) and waterproofing for real-world use. The purpose is to develop a near-room-temperature encapsulation that simultaneously offers ultra-low WVTR, high transparency, high curvature flexibility, and waterproof properties, enabling durable textile-integrated OLED displays.

Prior room-temperature encapsulation using CeO₂ and PDVB achieved WVTR of 1.81 g m⁻² day⁻¹ at 30 °C/100% RH, insufficient for OLED needs. A 2-dyad Al₂O₃/alucone encapsulation (50 °C) for perovskite solar cells reached 1.6 × 10⁻⁵ g m⁻² day⁻¹ at 30 °C/80% RH and maintained 96% PCE over 2000 h, but required long precursor purge times (>80 s), plasma treatment, and only demonstrated limited flexibility (5 mm radius, 500 cycles); waterproofing was not evaluated. Few reported encapsulations endure >1% bending strain or radius <5 mm, and reported waterproof times are typically <200 min. These gaps motivate a multifunctional barrier meeting OLED-grade permeability at near-room temperature with superior flexibility and water resistance.

- Encapsulation design: A bilayer thin-film encapsulation combining an inorganic Al₂O₃/TiO₂ nanolaminate (AT) deposited by atomic layer deposition (ALD) and an organic pV3D3 polymer capping layer deposited by initiated chemical vapor deposition (iCVD). - ALD optimization: TiO₂ deposited at near-room temperature (40–55 °C) using TDMAT and H₂O precursors to exploit a favorable ALD process window. WVTR and residual stress were measured versus deposition temperature to identify the optimal regime and understand desorption-induced stress at higher temperatures (>55 °C). Al₂O₃ was used as the transparent second laminate to form the AT nanolaminate for crack decoupling and improved transmittance. - Nanolaminate tuning: Sub-layer thicknesses (1.25–5 nm) and total thickness were varied at fixed total thickness (30 nm) to identify the minimum distinct layer thickness (≥3 nm) via HR-TEM and to maximize crack decoupling, lowering WVTR. Total thickness was optimized by balancing Fickian diffusion (favoring thicker films) with stress accumulation (increasing WVTR beyond ~50 nm), selecting 30 nm. - Stress compensation: Residual stress of the AT nanolaminate (tensile) and pV3D3 (compressive) was quantified, and pV3D3 thickness (85 nm) was chosen to drive the bilayer’s net residual stress toward zero using σ_total = Σ σ_i d_i. - Optical characterization: Transmittance of AT and bilayer was measured and modeled using Fresnel transfer matrix simulations to quantify improvements from the polymer capping layer. - Hydrophobicity and chemical stability: Water contact angle (WCA) measured for pV3D3; chemical robustness assessed by monitoring pV3D3 thickness during water immersion (up to 8 days) and FTIR spectra of the bilayer before/after 10-day immersion. - Permeation testing: WVTR measured at 30 °C/90% RH using an electrical calcium test; WVTR also tracked after bending and after prolonged water immersion (up to 7–10 days). - Mechanical reliability: Bending tests of encapsulation films (1000 cycles) at various strains to determine critical strain where WVTR degrades. OLED devices were bent 1000 times at 1.5 mm radius (≈1.7% tensile strain) to evaluate J–V–L stability and visual emission integrity. - Device fabrication: Red phosphorescent OLEDs on polyester textile with planarization and bottom encapsulation, device stack Al (100 nm)/Liq (1 nm)/Bebq₂:Ir(piq)₃ (70 nm)/NPB (62 nm)/MoO₃ (5 nm)/Ag (30 nm), followed by top encapsulation. Process temperatures were kept low (ALD ~40 °C; iCVD) to avoid substrate/device damage. - Reliability/lifetime: Operating lifetime measured at initial luminance 1000 nit under ambient conditions for bare vs encapsulated textile OLEDs. - Additional analyses: Residual stress vs deposition temperature, DFT-referenced reaction/desorption considerations from literature for TDMAT/H₂O ALD chemistry, TEM/EDS for nanolaminate visualization.

- Near-room-temperature ALD TiO₂ provides exceptional barrier performance: a 30 nm TiO₂ film deposited at 40 °C achieved WVTR of 3.17 × 10⁻⁵ g m⁻² day⁻¹ (30 °C, 90% RH), outperforming Al₂O₃ and even surpassing a 2.5-dyad Al₂O₃ multi-barrier deposited at 120 °C. - Unique temperature dependence: TiO₂ WVTR was lowest at 40–55 °C and increased sharply above ~55 °C due to precursor desorption, which elevated residual stress and promoted crack growth; TiO₂ residual stress was ~20 MPa at 40 °C vs 200–300 MPa at higher temperatures. - Nanolaminate optimization: An Al₂O₃/TiO₂ (AT) nanolaminate improved both barrier and optics. Sub-layer thickness of 3 nm yielded distinct layering and lowest WVTR (2.58 × 10⁻⁵ g m⁻² day⁻¹ at 30 nm total thickness). WVTR increased again when total thickness exceeded ~50 nm due to stress accumulation. - Optical transparency: Transmittance increased from 78.35% (single TiO₂) to 84.70% (AT) and to 87.93% for the bilayer after adding pV3D3 (consistent with Fresnel-based simulations). - Stress compensation and flexibility: AT nanolaminate exhibited tensile residual stress (~230.40 MPa); pV3D3 showed compressive residual stress (−77.78 MPa). With 85 nm pV3D3 on 30 nm AT, net residual stress approached zero, improving mechanical robustness. Critical strain increased from 0.84% (AT only) to ~2% (bilayer) after 1000 bending cycles, with minimal WVTR degradation. - Waterproofing: pV3D3 is hydrophobic (WCA 91°) and chemically stable in water; thickness unchanged after 8 days immersion and FTIR spectra unchanged after 10 days. Bilayer WVTR increased by only ~one order after 7 days in water, maintaining barrier integrity. - Device performance: Encapsulation extended textile OLED lifetime from 6 h (bare) to 160 h at 1000 nit under ambient conditions. After 1000 bends at 1.5 mm radius (~1.7% strain), J–V–L characteristics and lifetime were maintained. Devices continued to emit under folding/wrinkling and during/after 1440 min water immersion (no optoelectronic deterioration). - Process compatibility: Low-temperature encapsulation avoided thermal damage; devices retained optoelectronic characteristics post-encapsulation; textile substrates preserved their physical integrity (no rolling/shrinking/expansion).

The work demonstrates that a multifunctional, near-room-temperature encapsulation effectively addresses the core challenges for textile-integrated OLEDs: moisture/oxygen ingress, mechanical flexibility, and environmental (water) exposure. By exploiting the ALD process window for TiO₂ at 40–55 °C, the barrier achieves ultralow WVTR while minimizing thermal damage to OLEDs and textiles. The mechanistic link between elevated deposition temperature, precursor desorption, increased residual stress, and crack-driven permeation was validated by residual stress measurements and consistent with ALD reaction kinetics. Structurally, the Al₂O₃/TiO₂ nanolaminate maximizes crack decoupling and improves transparency, while the compressive, hydrophobic pV3D3 capping offsets tensile stress in the inorganic layers, yielding near-zero net stress for enhanced bendability and durable waterproofing. These materials and structural strategies translate directly to device-level gains: significantly extended operating lifetime at practical luminance, tolerance to high-curvature repetitive bending, and stable operation during prolonged water immersion. The results underscore TiO₂’s viability as a primary encapsulant at low temperatures and illustrate a generalizable stress-compensation and hydrophobic-capping approach for robust flexible electronics on textiles.

The study introduces a low-temperature bilayer encapsulation—an Al₂O₃/TiO₂ nanolaminate with a highly cross-linked pV3D3 capping layer—that enables reliable, foldable, and washable textile-based OLEDs. The barrier achieves WVTR on the order of 10⁻⁶ g m⁻² day⁻¹ at 30 °C/90% RH with high transparency (87.93%), near-zero residual stress after compensation, and superior flexibility and waterproofing. Applied to textile OLEDs, it extends ambient lifetime to 160 h at 1000 nit, preserves J–V–L under 1000 bends at 1.5 mm radius, and maintains performance after 1440 min water immersion. These findings position TiO₂ as a key low-temperature encapsulation material and provide a pathway to robust smart e-textiles. Potential future work includes: scaling ALD/iCVD processes for large-area or roll-to-roll manufacturing; comprehensive laundering and abrasion cycle testing on full garments; long-term operational stability studies under varying temperature/humidity extremes; integration with diverse device architectures and colors; and further optimization of barrier architectures for even lower WVTR and higher bend endurance.

- Lifetime enhancement, while substantial, was evaluated over 160 h at 1000 nit; longer-term (>1000 h) stability and operation under varied environmental conditions were not reported. - Mechanical reliability was demonstrated for 1000 bending cycles at 1.5 mm radius; broader cycling regimes (higher cycle counts, multiple radii/strains, torsion/stretch, folding fatigue) were not exhaustively assessed. - Waterproofing was validated by immersion tests (up to 7–10 days for barrier metrics and 1440 min for device operation), but standardized machine-laundering, detergent exposure, and abrasion/sweat chemistry tests were not included. - WVTR measurements were performed at 30 °C/90% RH via electrical calcium tests; cross-validation with alternative methods and performance at other temperatures/humidities were not presented. - Multi-affiliation and equal contribution notes aside, scalability and throughput of the combined ALD/iCVD processes for mass production were not experimentally addressed.